In recent years, there has been proposed an illumination apparatus with a visible light communication function to transmit a variety of information in a free space by using illumination light. Such illumination apparatus is disclosed in, e.g., Japanese Patent Application Publication No. 2011-34713. The illumination apparatus described in Japanese Patent Application Publication No. 2011-34713 includes a light emitting portion substrate on which light emitting diodes serving as semiconductor light emitting elements are disposed, a lighting circuit substrate which is connected to the light emitting portion substrate to control lighting of the light emitting diodes, and a visible light communication control substrate which performs a visible light communication control of the light emitting diodes. The visible light communication control substrate is detachably connected between the lighting circuit substrate and the light emitting portion substrate, and thus, in this conventional example, it is possible to generalize the design of the illumination apparatuses with/without the visible light communication function.
In the conventional example, the visible light communication is carried out by modulating an intensity of the illumination light based on an information signal (communication signal) to be transmitted. In other words, in an illumination light communication device on the transmitting side, the communication signal is superimposed on the illumination light by turning on/off an illumination light outputted from a light source unit including the light emitting diodes as a light source. Then, a receiver on the receiving side receives the communication signal by detecting a difference between a light intensity when the pulse of the communication signal is not superimposed and a light intensity when the pulse of the communication signal is superimposed.
In case of an amplitude control in which a dimming control of the light source unit is performed by varying the magnitude of a load current, the load current continuously flows without interruption. Thus, the communication signal also can be continuously superimposed. However, in case of a PWM control in which a dimming control of the light source unit is performed by alternately repeating a period during which a load current flows and a period during which no load current flows, the communication signal cannot be superimposed in the period during which no load current flows, and thus, there is a possibility that the visible light communication may be interrupted.